1. Field of the Invention
This invention relates to electronic cameras and, more particularly, to an electronic camera which compresses, frame by frame, image signals of a plurality of frames consecutively obtained in a snapshot or motion image recording mode thereby recording the compressed image signals onto a recording medium.
2. Description of the Prior Art
In the digital camera, when operating the shutter button, a subject image is taken by an image sensor, providing a camera signal. The obtained camera signal is subjected to signal processing, including color separation and YUV conversion. The image signal thus processed is thereafter recorded onto a recording medium through JPEG compression.
In the case that the recording medium is a detachable medium, say a memory card, there is a need to ensure beforehand the number of recordable frames or still pictures. For achieving this, the compression ratio upon JPEG compression must be controlled on each still image to thereby reduce the size of a compressed image down to a predetermined target size or the smaller.
Due to this, in a still image recording mode wherein one frame of still images is recorded in response to shutter button operation, an image signal is compressed with a predetermined compression ratio into a compressed image signal. Based on a size of the compressed image signal and a target size, calculated is an optimal compression ratio in order to compress the same image signal with an optimal compression ratio.
In a snapshot or motion image recording mode, however, the control of a compression ratio in a similar way takes a long time in recording one frame of image signals, hence decreasing the number of shootable/recordable frames per unit time, i.e. the number of frames. This, however, makes it difficult in a motion image recording mode to realize smooth reproduction of motion images. Also, in a snapshot recording mode, the interval of picture taking is increased resulting in an increased possibility of failing to shoot optimal images.
It is noted that, in image compression conforming to a JPEG format, the change of compression ratio is meant to change a Q factor, i.e. to change a quantization table. This technique is a well-known technology having been applied in most of the existing electronic cameras.